Real-time monitoring

ABSTRACT

Provided herein are methods for the real-time monitoring of an intracellular event or response. In particular, the methods provided herein monitor the conversion of a pro-substrate to a substrate for a protein sensor as a result of an intracellular event or response.

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/705,993 filed Sep. 26, 2012, which is herein incorporated byreference in its entirety.

FIELD

Provided herein are methods for the real-time monitoring of an event orresponse. In particular, the methods provided herein monitor theconversion of a stable pro-substrate to an unstable substrate for aprotein sensor, as a result of an event or response.

BACKGROUND

The monitoring of an intracellular, extracellular, or other event orresponse (e.g., reduction of metabolic activity, changes inintracellular radicals, induction of apoptosis, etc.) in response tovarious cellular insults brought on by chemical or biochemical compoundsis an important aspect for the study of diverse biological events.

SUMMARY

In some embodiments, the methods described herein comprise a noninvasiveapproach (e.g., bioluminescent assay) for continuous, real-timedetection and/or measurement of an event, response, condition, orsignal. In general, a stable pro-substrate (or one present in excess orcontinually added throughout the course of the assay) is continuouslyconverted into substrate in response to an event, agent, condition,etc., and the presence of the substrate is detected by its interactionwith a sensor (e.g., protein sensor). The generated substrate is rapidlyeliminated due to its inherent instability (or due to its rapidutilization by the protein sensor) resulting in conditions where nosignificant amount of the substrate is able to accumulate, allowingmeasurement of the event, response, condition, or signal in real-time ornear real-time, i.e., as they occur. Any changes in the activity of ameasured event result in the change of substrate production and,subsequently, in the change of the signal generated by the sensor andsubstrate.

Various embodiments of the invention are useful for detecting/monitoringintracellular and/or extracellular events, responses, conditions, orsignals. Detection of events, responses, conditions, or signals in celllysate or cell-free systems is also within the scope of the presentinvention. In some embodiments, the ability of the pro-substrate and/orsubstrate to enter and/or exit cells is tailored for the location (e.g.,intracellular, extracellular, cell-free) of the event, response,condition, or signal to be detected. In some embodiments, pro-substrateis converted into substrate intracellularly. In some embodiments,pro-substrate is converted into substrate extracellularly. In someembodiments, pro-substrate is converted into substrate in a cell lysate.In some embodiments, pro-substrate is converted into substrate in acell-free system. In some embodiments, pro-substrate and/or substratethat are capable of entering and/or exiting cells by any suitablemechanism are used in the detection of intracellular, extracellular,and/or cell-free events, responses, conditions, or signals. In someembodiments, pro-substrate and/or substrate that are not capable ofentering and/or exiting cells are used in the detection of extracellularand/or cell-free events, responses, conditions, or signals. Althoughmany of the embodiments described herein are described in the context ofcellular events, the compositions and methods are also applicable tocell lysates and/or cell-free systems. Many embodiments described hereinare described as applying to the detection of intracellular events;however, unless specifically noted as being for intracellular use onlyand not for extracellular or cell-free use, these embodiments are alsoapplicable to the monitoring of extracellular and/or cell-free events aswell.

An aspect of embodiments of the present invention, and one that is atleast partially responsible for the real-time capabilities of assaysutilizing this technology, is the continuous conversion of apro-substrate into a substrate in response to an event (e.g., anintracellular event, an extracellular event), and its continuouselimination due to substrate instability and/or rapidutilization/degradation of the substrate (e.g., relative to thepro-substrate, relative to the assay time-scale, relative to thetimescale of the events being monitored, etc.) by a protein sensor(e.g., a protein sensor present outside the cells). In certainembodiments, the substrate is rapidly utilized by the protein sensor. Inother embodiments, the substrate is inherently unstable (e.g.,chemically unstable, readily enzymatically degraded), and therefore doesnot accumulate. Rapid utilization, consumption, and/or degradation ofthe substrate results in little or no extracellular substrateaccumulation. Because the substrate does not accumulate and/or is onlypresent for a relatively short time (e.g., relative to thepro-substrate, relative to the assay time-scale, relative to thetimescale of the events being monitored, etc.), the signal generated bythe substrate and the protein sensor depends on the continuousproduction of substrate by a cellular event and is directly proportionalto the activity of measured intracellular events. In some embodiments,continuous production of substrate inside the cells (or extracellularly)and continuous usage of it by a protein sensor outside the cellsprovides an assay in which a steady state signal output level isachieved that reflects the magnitude, frequency, and or concentration ofthe event, response, condition or agent and changes primarily as afunction of changes in the levels thereof. In such embodiments, shouldthe rate of conversion of pro-substrate to substrate decrease (e.g., dueto slowing of the conversion, due to lack of availability ofintracellular pro-substrate), it is reflected in a real-time decrease insignal. Likewise, should the rate of conversion of pro-substrate tosubstrate increase (e.g., due to faster conversion rate, due toincreased availability of intracellular pro-substrate), it is reflectedin a real-time increase in signal. In some embodiments, continuousmonitoring of signal provides real-time readout of events, conditions,activities, etc.

In some embodiments, the signal generated at any given time isproportional to the event or response that is being monitored. In someembodiments, the generation of signal depends on measuring the activityof an enzyme or molecule that may be attributed to an intracellularevent or response in a living cell. In some embodiments, if the event,response or activity to be measured is inhibited or the cells are lysed,no signal (or reduced signal) is generated. In some embodiments, thegeneration of signal depends on measuring the activity of an enzyme ormolecule that may be attributed to an extracellular event or response.In such embodiments, if the event, response or activity to be measuredis inhibited or if export of key agents is inhibited, no signal (orreduced signal) is generated.

In some embodiments, a pro-substrate is continuously converted into asubstrate by a condition, event, response, or an agent that resultstherefrom. In some embodiments, a pro-substrate is continuouslyconverted into a substrate intracellularly, extracellularly, in celllysate, or in a cell-free system. In certain embodiments, thecharacteristics of the pro-substrate (e.g., concentration, stability,etc.) and/or assay conditions (e.g., amount of pro-substrate added,protein sensor concentration, number of pro-substrate additions, etc.)are selected/manipulated to ensure that sufficient concentration ofpro-substrate is present throughout the assay. Sufficient pro-substrateconcentration is a concentration that results in interaction of thepro-substrate with an intracellular event, condition, response, or agentassociated therewith when such an event, condition, response occurs. Insome embodiments, the pro-substrate is stable over the time-scale of theassay (e.g., T_(1/2)>assay run time). In some embodiments, pro-substrateconcentration is adjusted to compensate for any instability of thepro-substrate (e.g., greater pro-substrate concentration for a lessstable pro-substrate). In some embodiments, sufficient pro-substrate isprovided to ensure its conversion to the substrate, should the properintracellular condition, event, response, agent occur or be present. Insome embodiments, repeated additions of pro-substrate are made over thecourse of the assay. In some embodiments, pro-substrate is continuouslyflowed into the assay.

In some embodiments, the systems and methods provided herein comprise(a) a protein sensor (e.g., a luciferase enzyme) and (b) a pro-substrate(e.g., an entity that can be converted into a substrate for the proteinsensor). In embodiments of the present invention, a protein sensor andpro-substrate are introduced extracellularly. The pro-substrate is ableto move from the extracellular space into the interior of cells by anysuitable mechanism (e.g., diffusion, endocytosis, passive transport,active transport, etc.). In some embodiments, the pro-substrate is alsocapable of exiting the intracellular environment and returning to theextracellular space (e.g., by any suitable mechanism). Suitablepro-substrates include entities that are converted to substrate insidelive cells in response to a cellular event. The substrate moves out ofthe live cell and is utilized by the protein sensor placed outside thecells to produce a signal that can be measured and represents directmeasurement of an intracellular event.

In other embodiments, the extracellularly-introduced pro-substrate isincapable of moving into the interior of cells. In such embodiments, thepro-substrate is converted to substrate outside of cells in response toan event (e.g., extracellular, intracellular, etc.). In someembodiments, pro-substrate is converted to substrate upon cell export ofa particular agent. In some embodiments, the rate of conversion isdependent upon the amount of export of an agent. In these embodiments,the pro-substrate is introduced extracellularly, converted to substrateextracellularly, and utilized by the protein sensor extracellularly toproduce a signal that can be measured and represents direct measurementof an event or condition (e.g., extracellular, intracellular, etc.).

In yet other embodiments, the protein sensor and the pro-substrate areintroduced into a cell lysate or cell-free system. The pro-substrate isconverted into substrate upon the occurrence of an event or condition ofinterest and the substrate is utilized by the protein sensor. In suchembodiments, there is no need for the presence of cells.

Some suitable pro-substrates comprise a substrate moiety and afunctional moiety (or blocking moiety). In some embodiments, afunctional moiety (or blocking moiety) is an entity or functional group(e.g., peptide, organic molecule, etc.) that can be severed from thesubstrate moiety by an event (e.g., enzymatic cleavage, chemicalcleavage, etc.). When the functional moiety is removed from thesubstrate moiety (e.g., by an intracellular event) the substrate for theprotein sensor is liberated.

Other suitable pro-substrates are incomplete substrates (e.g.,pro-substrate becomes a substrate upon addition of a moiety orfunctional group). In certain embodiments, a functional moiety is anentity (e.g., peptide, organic molecule, etc.) that is added to thepro-substrate by an event (e.g., chemical reaction, enzymaticallyfacilitated addition, etc.), to produce a substrate.

In various embodiments, the substrate (e.g., liberated from the blockingmoiety, modified by an intracellular event, etc.) is free to interactwith the protein sensor (e.g., in the extracellular space, in theintracellular space). Upon exposure of the substrate (e.g., freed fromthe functional element, modified from its “pro” form, etc.) to theprotein sensor, the substrate is utilized by the protein sensor toproduce a detectable signal (e.g., luminescence). In some embodiments,the substrate is altered, consumed, and/or degraded by the proteinsensor such that it cannot again be utilized by a sensor protein in thesignal-producing reaction. The alteration, consumption, and/ordegradation of the substrate by the protein sensor results in noaccumulation of the substrate.

In some embodiments, the substrate is rapidly degraded (e.g.,chemically, enzymatically, etc.), consumed (e.g., by the proteinsensor), or otherwise eliminated following its generation by theinteraction of the pro-substrate with the agent associated with theresponse, event, or condition of interest. The instability of thesubstrate can result from one or a combination of: the inherentinstability of the substrate, susceptibility of the substrate todegradation or modification, utilization of the substrate by the proteinsensor, instability of the substrate under physiological and/orintracellular conditions, etc. As a result, little to no (e.g.,substantially none) substrate accumulates (e.g., intracellularly,extracellularly, etc.) during the course of an assay.

In some embodiments, a protein sensor is an enzyme. In some embodiments,a protein sensor produces a detectable signal upon interaction with asubstrate. The protein sensor is present in an amount required for rapidsubstrate utilization. A protein sensor retains activity outside thecell over the time-scale of the assay. In some embodiments, a proteinsensor is a luciferase, chromophoric protein, fluorescent protein, etc.In some embodiments, the protein sensor is a luciferase selected fromthose found in Omphalotus olearius, fireflies (e.g., Photinini), Renillareniformis, mutants thereof, portions thereof, variants thereof, and anyother luciferase enzymes suitable for the systems and methods describedherein. In some embodiments, the protein sensor is a modified, enhancedluciferase enzyme from Oplophorus (e.g., NANOLUC enzyme from PromegaCorporation, SEQ ID NO: 1 or a sequence with at least 70% identity(e.g., >70%, >80%, >90%, >95%) thereto). In some embodiments, theprotein sensor is a thermostable Photuris pennsylvanica luciferase(e.g., SEQ ID NO: 2 or a sequence with at least 70% identity(e.g., >70%, >80%, >90%, >95%) thereto).

In some embodiments, a pro-substrate is provided, introduced, and/oradministered to a sample and/or the extracellular space. In someembodiments, a pro-substrate is converted to a substrate for a proteinsensor by an enzyme of interest, target enzyme, or other agent. In someembodiments, a pro-substrate comprises a substrate moiety (e.g.,substrate for the protein sensor) and an additional moiety (e.g.,functional moiety, blocking moiety, etc.), wherein the additional moietyis removed from the pro-substrate (e.g., by an enzyme of interest) toproduce a substrate for the protein sensor. In some embodiments, apro-substrate lacks a functional group or chemical moiety of a wholesubstrate and addition of that group or moiety to the pro-substrateconverts the pro-substrate into a substrate. In some embodiments,chemical and/or enzymatic modification of the pro-substrate results inthe formation of the substrate. In some embodiments, the pro-substratestructure is converted into a substrate structure (e.g., chemically,enzymatically, etc.). In some embodiments, the substrate is a substratefor a luciferase enzyme. In some embodiments, the pro-substrate is apro-substrate for a luciferase enzyme. In some embodiments, thepro-substrate is a pro-substrate for a modified, enhanced luciferaseenzyme from Oplophorus, e.g., NanoLuc enzyme from Promega Corporation(e.g., SEQ ID NO: 1). In some embodiments, the pro-substrate comprisescoelenterazine, a coelenterazine derivative, a structural or functionalequivalent of coelenterazine, a molecule substantially equivalent tocoelenterazine (e.g., structurally and/or functionally), or moleculefunctionally or structurally similar to coelenterazine. In someembodiments, the pro-substrate comprises Formula I:

wherein R comprises an organic moiety, peptide, amino acid, nucleotide,or nucleic acid that can be acted upon, e.g., removed or modified, by anenzyme of interest. The above is an exemplary pro-substrate thatprovides a model for other pro-substrates for othercoelenterazine-utilizing luciferases, other luciferase, or otherenzymes.

In some embodiments, upon being acted on by the enzyme of interest orother agent (e.g., intracellularly, extracellular, in a cell-freeenvironment, etc.), the pro-substrate is converted to a substrate forthe protein sensor. In some embodiments, the substrate diffuses from thecell into the extracellular space (e.g., media, etc.). In someembodiments, the pro-substrate is added to living cells. In someembodiments, the pro-substrate and the protein sensor are addedsimultaneously. In some embodiments, the pro-substrate and the proteinsensor are added at different times. In some embodiments, thepro-substrate is stable for an extended period of time (e.g., >1hour, >2 hours, >6 hours, >12 hours, >24 hours, >48 hours, >72hours, >100 hours). In some embodiments, the protein sensor is stablefor an extended period of time (e.g., >1 hour, >2 hours, >6 hours, >12hours, >24 hours, >48 hours, >72 hours, >100 hours).

In some embodiments, the method disclosed herein can be used forstudying diverse biological events, including but not limited to:metabolic activity of live cells, apoptosis by measuring caspaseactivation, generation of reactive oxygen species by measuring hydrogenperoxide formation, reducing capacity of the cell, oxidative capacity ofthe cell, etc. In some embodiments, a biological event or intracellularevent, response, or condition (e.g., reducing capacity, oxidativecapacity, etc.) may be attributed to a particular enzyme or molecule,e.g., protein. In some embodiments, the enzyme comprises a caspase, aprotease, a cytochrome P450 (CYP450) enzyme, or monoamine oxidase (MAO),reductase, dehydrogenase, oxidoreductase, enzymes involved in oxidativephosphorylation, enzymes involved in glycolysis, etc. In someembodiments, the method described herein allows for the detection,measurement, e.g., quantitation, of the enzyme or molecule, e.g.,protein or reactive oxygen species.

Embodiments of the present invention find use in any suitable livingcells, including but not limited to bacterial cells (e.g., E. coli),eukaryotic cells (e.g., X. laevis, C. elegans, S. cerevisiae), andmammalian cells (e.g., non-human primates, chicken, mouse, human, etc.).In some embodiments, cells used in embodiments described herein are incultured cells, in vitro, in situ, or in vivo. In some embodiments,pro-substrate conversion to substrate and substrate utilization by theprotein sensor both take place outside of cells (e.g., extracellularly)or in an environment lacking living cells (e.g., cell lysate, cell-freesystem, etc.).

In some embodiments, the substrate is unstable (e.g., inherentlyunstable, rapidly degraded by the sensor protein). In some embodiments,the instability of the substrate allows for the real-time monitoring ofan event or response (e.g., intracellular event or response). Theinstability of the substrate prevents substrate accumulation such that asteady state signal output level is achieved that reflects the presenceof the agent of interest and changes primarily as a function of changesin the concentration of activity of the agent of interest. Theinstability of the substrate may be derived from the chemicalinstability of the substrate and/or the rapid consumption of thesubstrate by a protein sensor. Because of the instability of thesubstrate, the signal generated by a protein sensor necessarily resultsfrom a recently converted substrate (e.g., converted frompro-substrate). In some embodiments, the pro-substrate is stable. Insome embodiments, processing of the pro-substrate (e.g., by live cells)creates an unstable active substrate for the protein sensor. In someembodiments, an intracellular event or response (e.g., directly orindirectly resulting in conversion of the pro-substrate into thesubstrate) can be monitored over an extended period of time due to thestability of both the pro-substrate and the protein sensor (e.g.,stability of the pro-substrate inside and/or outside the cell and/orstability of the protein sensor outside the cell).

In some embodiments, the method described herein does not requireengineering of a living cell (e.g., the protein sensor does not need tobe expressed by the living cell). In some embodiments, the proteinsensor does not enter the cell. In some embodiments, the methoddescribed herein can be used with different pro-substrates, e.g.,pro-furimazine, pro-coelenterazine, pro-coelenterazine derivative oranalog, or pro-luciferin, and different protein sensors, e.g., differentluciferase enzymes, e.g., luciferase enzymes from Oplophorus, Photinuspyralis, Photuris pennsylvanica, Renilla, or Gaussia. In someembodiments, the luciferase enzyme may be modified, e.g., have enhancedbioluminescence, enhanced signal stability, or is thermostable. In someembodiments, the method described herein can be used in differentbiological systems, e.g., bacteria, yeast, eukaryotic, e.g., mammalian,cells, tissues, and animal models. In some embodiments, the methodsdescribed herein can be used to monitor, in real-time, an intracellularevent or response in vivo, e.g., inside a cell(s), tissue or organ of aliving animal, e.g., mouse, rat, or human.

In some embodiments, the present invention provides methods of detectingan intracellular event, condition, or response comprising: (a)providing: (i) a cell; (ii) a protein sensor, wherein the protein sensorgenerates a detectable signal upon interaction with a substrate, and(iii) a stable pro-substrate, wherein the pro-substrate is capable ofentering the cell, wherein interaction of the pro-substrate with anagent associated with the intracellular event, condition, or responseconverts the pro-substrate into the unstable substrate for the proteinsensor; (b) detecting the signal generated by the interaction of thesubstrate and the protein sensor, wherein the presence of the signalindicates the occurrence of the intracellular event, condition, orresponse in real time, wherein the absence of an agent associated withthe intracellular event, condition, or response results in the real-timeabsence of the signal, and wherein the magnitude of the signalcorrelates with the amount of the agent.

In some embodiments, the present invention provides methods of detectingan event, condition, or response comprising: (a) providing: (i) a cell;(ii) a protein sensor, wherein the protein sensor generates a detectablesignal upon interaction with a substrate, and (iii) a stablepro-substrate, wherein the pro-substrate is incapable of entering thecell, wherein interaction of the pro-substrate with an agent associatedwith the event, condition, or response converts the pro-substrate intothe unstable substrate for the protein sensor; (b) detecting the signalgenerated by the interaction of the substrate and the protein sensor,wherein the presence of the signal indicates the occurrence of theevent, condition, or response in real time, wherein the absence of anagent associated with the extracellular event, condition, or responseresults in the real-time absence of the signal, and wherein themagnitude of the signal correlates with the amount of the agent. In someembodiments, the event, condition, or response is extracellular orintracellular.

In some embodiments, the present invention provides methods ofextracellularly detecting an event, condition, or response comprising:(a) providing: (i) a cell; (ii) a protein sensor, wherein the proteinsensor generates a detectable signal upon interaction with a substrate,and (iii) a stable pro-substrate, wherein the pro-substrate is incapableof entering the cell, wherein interaction of the pro-substrate with anagent associated with the event, condition, or response converts thepro-substrate into the unstable substrate for the protein sensor; (b)detecting the signal generated by the interaction of the substrate andthe protein sensor, wherein the presence of the signal indicates theoccurrence of the event, condition, or response in real time, whereinthe absence of an agent associated with the extracellular event,condition, or response results in the real-time absence of the signal,and wherein the magnitude of the signal correlates with the amount ofthe agent. In some embodiments, the present invention provides methodsof extracellularly detecting an event, condition, or responsecomprising: (a) providing: (i) a cell; (ii) a protein sensor, whereinthe protein sensor generates a detectable signal upon interaction with asubstrate, and (iii) a stable pro-substrate, wherein the pro-substrateis not converted to substrate intracellularly, wherein interaction ofthe pro-substrate with an agent associated with the event, condition, orresponse converts the pro-substrate into the unstable substrate for theprotein sensor; (b) detecting the signal generated by the interaction ofthe substrate and the protein sensor, wherein the presence of the signalindicates the occurrence of the event, condition, or response in realtime, wherein the absence of an agent associated with the extracellularevent, condition, or response results in the real-time absence of thesignal, and wherein the magnitude of the signal correlates with theamount of the agent.

In some embodiments, the present invention provides methods of detectingan event, condition, or response comprising: (a) providing: (i) a celllysate or cell-free media; (ii) a protein sensor, wherein the proteinsensor generates a detectable signal upon interaction with a substrate,and (iii) a stable pro-substrate, wherein interaction of thepro-substrate with an agent associated with the event, condition, orresponse converts the pro-substrate into the unstable substrate for theprotein sensor; (b) detecting the signal generated by the interaction ofthe substrate and the protein sensor, wherein the presence of the signalindicates the occurrence of the event, condition, or response in realtime, wherein the absence of an agent associated with the extracellularevent, condition, or response results in the real-time absence of thesignal, and wherein the magnitude of the signal correlates with theamount of the agent.

In some embodiments, detecting comprises real-time monitoring. In someembodiments, the substrate is unstable. In some embodiments, thesubstrate does not accumulate (e.g., extracellularly) following theinteraction of the pro-substrate with the agent associated with theevent, condition, or response. In some embodiments, substantially all ofthe substrate generated by interaction of the pro-substrate with theagent associated with the event, condition, or response is eitherdegraded or utilized by the protein sensor.

In some embodiments, substantially all (e.g., all detectable amountsunder assay detection conditions) of the substrate generated byinteraction of the pro-substrate with the agent associated with theevent, condition, or response (e.g., intracellular event, condition, orresponse) is either degraded or utilized by the protein sensor within atime-frame that is useful for real-time or near real-time detection ofthe interaction (e.g., depending upon the type of assay, the length ofthe experiment, and the event or condition being monitored, substratemay be degraded or utilized within 1 second, 2 seconds, 5 seconds, 10seconds, 20 seconds, 1 minute, 2 minutes, 5 minutes, 10 minutes, 20minutes, or more time following substrate generation). In someembodiments, substantially all of the substrate generated by interactionof the pro-substrate with the agent associated with the intracellularevent, condition, or response is either degraded or utilized by theprotein sensor within 1 minute of the interaction. In some embodiments,substantially all of the substrate generated by interaction of thepro-substrate with the agent associated with the intracellular event,condition, or response is either degraded or utilized by the proteinsensor within 30 seconds of the interaction. In some embodiments,substantially all of the substrate generated by interaction of thepro-substrate with the agent associated with the intracellular event,condition, or response is either degraded or utilized by the proteinsensor within 10 seconds of the interaction. In some embodiments, thesubstrate is a real-time substrate.

In some embodiments, the signal generated by the interaction of thesubstrate and the protein sensor only persists when substrate iscontinually produced by interaction of the pro-substrate with the agentassociated with the intracellular event, condition, or response. In someembodiments, increases and decreases in the concentration of the agentassociated with the intracellular event, condition, or response resultin corresponding increases and decreases in the signal generated by theinteraction of the substrate and the protein sensor. In someembodiments, changes in the signal occur within 5 minutes of changes inagent concentration. In some embodiments, changes in the signal occurwithin 1 minute of changes in agent concentration. In some embodiments,changes in the signal occur within 30 seconds of changes in agentconcentration. In some embodiments, changes in the signal occur within10 seconds of changes in agent concentration. In some embodiments, thesignal is a real-time signal. In some embodiments, the signal is anon-accumulated signal.

In certain embodiments, the pro-substrate enters cells at a certain rate(e.g., constant or varied by an operator or experimenter over the courseof an assay). In certain embodiments, the pro-substrate is convertedinto substrate at a certain rate (e.g., constant or varied over thecourse of an assay). In some embodiments, the rate of pro-substrateentry is sufficient to provide a pool of intracellular pro-substrate forconversion into substrate at whatever rate the cell is capable of Insome embodiments, pro-substrate entry into cells is not rate limiting tothe production of signal from substrate. In some embodiments, substrateexit from the cell is not rate limiting to the production of signal. Insome embodiments, conversion of pro-substrate to substrate is ratelimiting to the production of signal. In some embodiments, theconversion of pro-substrate to substrate acts as the ‘gatekeeper’ (orlimiting factor), determining the amount of substrate that is madeavailable for detection. In such embodiments, conversion frompro-substrate to substrate is regulated by cellular conditions,activities and/or events. When events, conditions, or activities raisethe rate of conversion, the amount of substrate, and therefore theamount of signal produced increases. When events, conditions oractivities slow the rate of conversion (or stop conversion), the amountof substrate, and therefore the amount of signal produced decreases. Insuch embodiments, the rate of conversion of pro-substrate to substrateand not the rate of pro-substrate entry or substrate exit is controlling(e.g., allowing the monitoring of events/conditions that affect the rateof pro-substrate conversion). The substrate does not accumulateintracellularly, for example, because the substrate is degraded orutilized before any such accumulation can occur. If the cell fails toconvert pro-substrate into substrate or slows its conversion (e.g., dueto cellular conditions or events), the signal detected decreases. Whencells more rapidly convert pro-substrate to substrate (e.g., due tocellular conditions or events), the signal detected increases. Intechnologies where substrate accumulates, even if conversion ofpro-substrate ceases, the signal persists. In embodiments of the presentinvention, increases or decreases in signal are directly attributable tocorresponding increases or decreases in the amount of substrate producedand the cellular activity or agent responsible for the conversion of thepro-substrate.

In certain embodiments, methods provided herein allow for thepinpointing of maximal effect, minimal effect, and/or transition pointsof the studied event, response, or condition. Among other advantages ofsuch an assay, assays with such functionality allow experimenters todesign functional end point assays which occur at the most optimal time.

In some embodiments, the protein sensor is provided extracellularly. Insome embodiments, the protein sensor is incapable of entering the cell.In some embodiments, the interaction of the substrate and the proteinsensor occurs extracellularly.

In some embodiments, the protein sensor comprises a luciferase enzyme.In some embodiments, the luciferase enzyme comprises an Oplophorus,beetle, Renilla or Gaussia luciferase. In some embodiments, theluciferase enzyme comprises a luciferase with enhanced protein stabilityenhanced bioluminescence and/or enhanced signal stability. In someembodiments, the luciferase enzyme comprises an Oplophorus luciferasewith enhanced protein stability enhanced bioluminescence and/or enhancedsignal stability. In some embodiments, the luciferase enzyme comprises abeetle luciferase selected from Photinus pyralis or Photurispennsylvanica. In some embodiments, the pro-substrate comprises apro-furimazine, pro-furimazine derivative, pro-coelenterazine,pro-coelenterazine derivative or analog, pro-luciferin, or pro-luciferinderivative.

In some embodiments, the agent (e.g., agent responsible for theconversion of pro-substrate to substrate) is an enzyme or molecule, thepresence of which correlates to the intracellular event or response. Insome embodiments, the enzyme or molecule comprises a caspase, aprotease, a reactive oxygen species, a cytochrome P450 (CYP450) enzyme,or monoamine oxidase (MAO), reductase, dehydrogenase, or oxidoreductase.

In some embodiments, the present invention provides kits for detectingan intracellular event or response, the kit comprising: (a) a proteinsensor, wherein the protein sensor generates a detectable signal uponinteraction with a substrate, and (b) a pro-substrate, wherein thepro-substrate is capable of entering a cell, wherein interaction of thepro-substrate with an agent associated with the intracellular event,condition, or response converts the pro-substrate into a substrate forthe protein sensor. Any of the compositions useful in carrying outembodiments of the present invention (e.g., protein sensor,pro-substrates, buffers, cells, controls, etc.), alone or in combinationwith other compositions, may be provided in the form of a kit. Forexample, protein sensor and pro-substrate may be provided in a kit fordetection of a cellular condition (e.g., cell viability). Kits mayfurther comprise appropriate controls (e.g., negative, positive),containers, assay reagents, and/or detection reagents.

In certain embodiments, the signal is detected by any suitable device,detector, apparatus, system, etc. Depending upon the detectable signal(e.g., luminescence, colorimetric, etc.), and/or the assay conditions(e.g., multiplex, cell type, etc.), suitable detectors may include, butare not limited to: CCD cameras, video cameras, photographic film, laserscanning devices, fluorometers, photodiodes, quantum counters,epifluorescence microscopes, scanning microscopes, flow cytometers,fluorescence microplate readers, or a fluorescence microscope. In someembodiments, detection is by real-time monitoring, end-point reads,sequential, continuous reads (e.g., every: 1 second, 2 seconds, 5seconds, 10 seconds, 30 seconds, 1 minute, 2 minutes, etc.), or anyother suitable detection/monitoring scheme.

In some embodiments, a computer-based analysis program is used tocorrelate the raw data generated by the detection assay (e.g., thepresence, absence, or amount of signal) into the concentration of anagent of interest or the magnitude, frequency and/or concentration of anevent, condition, and or response related to that agent. The data may bereported in any suitable format (e.g., raw signal data, convertedsignal, agent concentration, event frequency, cellular conditions,etc.). The data may be stored (e.g., in a computer-readable format),displayed (e.g., on a monitor), printed, reported, etc. In someembodiments, methods provide computer-assisted analysis of the data(e.g., raw or correlated).

In some embodiments, the present invention provides methods of detectingan event, condition, or response comprising: detecting a signalgenerated by the interaction of a substrate and a protein sensor;wherein the substrate is the product of an interaction of thepro-substrate with an agent associated with the event, condition, orresponse; wherein the presence of the signal indicates the occurrence ofthe event, condition, or response in real time; wherein the absence ofan agent associated with the event, condition, or response results inthe real-time absence of the signal; and wherein the magnitude of thesignal correlates with the amount of the agent. In some embodiments, theevent, condition, or response is intracellular, extracellular, orcell-free. In some embodiments, detecting comprises real-timemonitoring. In some embodiments, detecting comprises end-pointdetection. In some embodiments, detecting comprises continuousmonitoring of the signal. In some embodiments, detecting comprisescontinuous monitoring of intracellular events in live cells overextended period of time (e.g., 10 minutes . . . 20 minutes . . . 1 hour. . . 2 hours . . . 4 hours . . . 12 hours . . . 24 hours . . . 48 hours. . . 72 hours, 1 week, 4 weeks, or more). In some embodiments, thepro-substrate is stable and continuously enters the cell. In someembodiments, the pro-substrate is converted to a substrate by anintracellular event, condition or response and continuously exits thecell. In some embodiments, the substrate generates a signal byinteraction with a protein sensor placed outside the cell. In someembodiments, the substrate is unstable (e.g., degraded and or utilizedin <30 seconds, <1 minute, <2 minutes, <5 minutes, <10 minutes, <30minutes, <1 hours, or any other timescale that is rapid with respect tothe timescale of the experiment or assay). In some embodiments, thesubstrate does not accumulate (e.g., extracellularly) following theinteraction of the pro-substrate with the agent associated with theintracellular event, condition, or response. In some embodiments,substantially all of the substrate generated by interaction of thepro-substrate with the agent associated with the intracellular event,condition, or response is either degraded or utilized by the proteinsensor. In some embodiments, the signal produced by the substrate andprotein sensor depends on a continuous supply of substrate by a livecell. In some embodiments, the substrate is a real-time substrate. Insome embodiments, the signal generated by the interaction of thesubstrate and the protein sensor only persists when substrate iscontinually produced by interaction of the pro-substrate with the agentassociated with the intracellular event, condition, or response. In someembodiments, increases and decreases in the concentration of the agentassociated with the intracellular event, condition, or response resultin corresponding increases and decreases in the signal generated by theinteraction of the substrate and the protein sensor. In someembodiments, the signal is a real-time signal. In some embodiments, thesignal is a non-accumulated signal. In some embodiments, the substrateis capable of exiting the cell. In some embodiments, the protein sensoris provided extracellularly. In some embodiments, the protein sensor isincapable of entering the cell. In some embodiments, the interaction ofthe substrate and the protein sensor occurs extracellularly. In someembodiments, the live cells are not engineered to express the proteinsensor. In some embodiments, the pro-substrate, substrate or proteinsensor are non-invasive and do not interfere with cellular function. Insome embodiments, the protein sensor retains activity over the timescale of the assay. In some embodiments, the protein sensor comprises aluciferase enzyme. In some embodiments, the luciferase enzyme comprisesan Oplophorus, beetle, Renilla or Gaussia luciferase. In someembodiments, the luciferase enzyme comprises an Oplophorus luciferasewith enhanced protein stability, enhanced bioluminescence, and/orenhanced signal stability. In some embodiments, the luciferase enzymecomprises a beetle luciferase selected from Photinus pyralis or Photurispennsylvanica. In some embodiments, the pro-substrate comprises apro-furimazine, pro-furamizine derivative, pro-coelenterazine,pro-coelenterazine derivative or analog, pro-luciferin, or pro-luciferinderivative. In some embodiments, the agent is an enzyme or molecule, thepresence of which correlates to the intracellular event or response. Insome embodiments, the enzyme or molecule comprises a caspase, aprotease, a reactive oxygen species, a cytochrome P450 (CYP450) enzyme,or monoamine oxidase (MAO), reductase, dehydrogenase, or oxidoreductase.In some embodiments, intracellular event, condition, or response isdetected in bacterial cells. In some embodiments, the intracellularevent, condition, or response is detected in eukaryotic cells (e.g.,mammals, fish, birds, worms insects yeast, etc.). In some embodiments,the intracellular event, condition, or response is detected in mammaliancells (e.g., human, non-human primate, feline, canine, bovine, equine,prcine, rodent, etc.). In some embodiments, the intracellular event,condition, or response occurs is detected in a tissue or whole organism.In some embodiments, multiple different intracellular events,conditions, or responses are detected simultaneously in a multiplexassay or format.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the scheme of an assay for the real-time noninvasivecontinuous monitoring of intracellular events using embodiments of themethods described herein.

FIG. 2 illustrates that conversion of the pro-substrate to a substrateby live cells can be monitored outside the cells using the compositiondescribed herein.

FIG. 3 illustrates that the signal generated using the method describedherein is proportional to the number of live cells.

FIG. 4 illustrates that a rapid decrease in signal can be measured uponcell death using the method described herein.

FIG. 5 illustrates that cell death can be monitored in real-time usingthe method described herein.

FIG. 6 illustrates that the ability of the method described herein tomeasure changes in real-time is a property not reported by otherconventional assays.

FIG. 7 illustrates that the pro-substrate and protein sensor do notinterfere with cell viability.

FIG. 8 illustrates the real-time dynamic monitoring of cellproliferation using the method described herein.

FIG. 9 illustrates the continuous real-time monitoring of inducedchanges in cell viability using the method described herein.

FIG. 10 illustrates the monitoring of the pharmacological response inreal-time using the method described herein.

FIG. 11 illustrates determining the optimal timing of pharmacologicalresponses using the method described herein.

FIG. 12 illustrates that optimal time of responses depends on cell typeusing the method described herein.

FIG. 13 illustrates the measurement of time-dependent changes in drugIC₅₀ values using the same set of samples in the method describedherein.

FIG. 14 illustrates responses determined using the method describedherein are in agreement with other conventional methods.

FIG. 15 illustrates the use of real time cell viability measurements fordetermining the optimal time points to perform end point downstreamanalysis.

FIG. 16 illustrates the ability of the method described hereincomprising a NanoLuc protein sensor and pro-furimazine substrate(PBI-4600) to be multiplexed with another luminescent assay, CASPASE-GLO3/7 assay (“Caspase”)

FIG. 17 illustrates the method described herein can be multiplexed witha reporter gene assay allowing normalization of the expression levels ofthe reporter to the number of live cells present in the sample.

FIG. 18 illustrates that using the method described herein allowsluminescence assays to be multiplexed with fluorescent assays in livecells.

FIG. 19 illustrates the real-time monitoring of drug effect on cellviability over an extended period of time.

FIG. 20 illustrates that the method described herein can be used in anendpoint format.

FIG. 21 illustrates the stability of the pro-furimazine substrate,PBI-4600.

FIG. 22 illustrates the stability of the NANOLUC protein sensor in cellculture conditions for at least 72 hours.

FIG. 23 illustrates real-time monitoring of cell proliferation usingPBI-4601, a pro-coelenterazine substrate

FIG. 24 illustrates real-time measurement of caspase activation usingthe method described herein comprising a NanoLuc protein and apro-furimazine caspase probe (PBI-3741).

FIG. 25 illustrates the real-time monitoring of reactive oxygen species(ROS) in A549 cells using the method described herein comprising aNanoLuc protein sensor and a pro-furimazine hydrogen peroxide substrate(PBI-4759).

FIG. 26 illustrates the real-time monitoring of bacterial growth usingthe method described herein.

DEFINITIONS

As used herein, the term “enzyme or molecule of interest” refers to anenzyme or molecule, e.g., protein, whose activity may be attributed toan event or response.

As used herein, the term “agent” refers to an enzyme or molecule whosepresence correlates to a condition, event or response.

As used herein, the term “pro-substrate” refers to a molecule that canbe converted by an enzyme or molecule of interest to a substrate for aprotein sensor. As used herein, the term “pro-furimazine substrate”refers to a pro-substrate that can be converted to furimazine (e.g.,incomplete furimazine, modified furimazine, complete furimazine, etc.).As used herein, the term “pro-coelenterazine” refers to a pro-substratethat can be converted to coelenterazine (e.g., incompletecoelenterazine, modified coelenterazine, complete coelenterazine, etc.).As used herein, the term “pro-luciferin” refers to a pro-substrate thatcan be converted to luciferin (e.g., incomplete luciferin, modifiedluciferin, complete luciferin, etc.).

As used herein, the term “converted substrate” refers to a molecule thatis a substrate for a protein sensor and was derived from apro-substrate.

As used herein, the term “protein sensor” refers to an enzyme or proteinthat utilizes a substrate acted upon by an enzyme or molecule ofinterest to generate a detectable and/or measureable signal.

As used herein, the term “real-time” refers to an assay in which thetiming of a detectable signal is correlated to the timing of an event,response, condition or agent of interest at multiple time points. Thesignal is detected and/or measured either substantially concurrentlywith the presence and/or occurrence of the event, response, condition,or agent of interest or after a short lag time. A lag or delay betweenthe timing of an event, response, condition, or agent of interest andthe detectable signal is, for example, the result of the time requiredfor the cellular and/or assay steps necessary for detection. Preferablysuch lag or delay is less than 30 minutes, e.g., 25 minutes, 20 minutes,15 minutes, 10 minutes, 5 minutes, 4 minutes, 3 minutes, 2 minutes, 1minute or less, including no lag or delay, or any time period that isshort (e.g., <10%, <5%, <2%, <1%) of the time period being monitored.Due to consistent lag times over the time course of an assay, thedelayed signal is still suitable to provide real-time readout of thetiming of an event, response, condition or agent of interest.

As used herein, the term “real-time substrate” refers to a substratethat is sufficiently unstable and/or accumulated in low enoughconcentration that substantially all of the signal detected from theinteraction of the substrate with a corresponding protein sensor(e.g., >90%, >91%, >95%, >98%, >99%, >99.9%) is the result of substrategenerated or added to the system in real-time (e.g., within thepreceding 10 minutes, within the preceding 5 minutes, within thepreceding 1 minute, within the preceding 30 seconds, within thepreceding 10 seconds, within the preceding 5 seconds, within thepreceding 1 second, or within a time that is <1% of totalexperiment/assay time) and not from substrate accumulated from earlieraddition or generation.

As used herein, the term “real-time signal” refers to a detectablesignal (e.g., light emission) that is directly proportional to orcorrelated to an event, condition, response, or agent, such thatincreases in the frequency, magnitude, or concentration of the event,condition, response, or agent result in detectable changes (e.g.,increase or decrease) in signal in real-time (e.g., within 10 minutes,within 5 minutes, within 1 minute, within 30 seconds, within 10 seconds,within 5 seconds, within 1 second, or within a time that is <1% of totalexperiment/assay time), and decreases in the frequency, magnitude, orconcentration of the event, condition, response, or agent result indetectable changes (e.g., increase or decrease) in signal in real-time(e.g., within 1 hour, within 30 minutes, within 20 minutes, within 10minutes, within 5 minutes, within 1 minute, within 30 seconds, within 10seconds, within 5 seconds, within 1 second).

As used herein, the term “accumulated signal” refers to a detectablesignal that, at any given point in time, correlates in magnitude to thesum of the frequencies, magnitudes, or concentrations of events,conditions, responses, or agents over a span of time (e.g., the timecourse of the assay). The accumulated signal increases at a ratesubstantially proportional to the frequency, magnitude, or concentrationof an event, condition, response or agent. In the absence of an event,condition, response, or agent, the accumulated signal remainssubstantially constant.

As used herein, the term “non-accumulated signal” refers to a detectablesignal that, at any given point in time, correlates in magnitude to thefrequency, magnitude, or concentration of an event, condition, responseor agent at a given point in time. The non-accumulated signal increasesand decreases as the frequency, magnitude, or concentration of an event,condition, response, or agent increases and decreases. Due to the timerequired for a signal to be generated following the presence oroccurrence of the event, condition, response or agent, there may be alag time between the signal and the event, condition, response, or agentto which it correlates. Depending upon the assay conditions, lag timesmay be about 10 minutes, about 5 minutes, about 1 minute, about 30seconds, about 10 seconds, about 5 seconds, about 1 second, less than 1second, times therein, or a time that is <1% of total experiment/assaytime. Consistent lag times over the course of an assay allow real-time,continuous monitoring of the frequency, magnitude, or concentration ofan event, condition, response, or agent over a span of time using anassay that reports a non-accumulated signal.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to skill in theart, may occur in amounts that do not preclude the effect thecharacteristic was intended to provide.

As used herein, term “luciferin” refers to a molecule of Formula II:

Entities such as “modified luciferin,” “pro-luciferin,” “a luciferinmoiety,” etc. are variants of the above structure.

As used herein, term “coelenterazine” refers to a molecule of FormulaIII:

Entities such as “modified coelenterazine,” “pro-coelenterazine,” “acoelenterazine moiety,” etc. are variants of the above structure.

As used herein, the term “furimazine” refers to a molecule of FormulaIV:

Entities such as “modified furimazine,” “pro-furimazine,” “a furimazinemoiety,” etc. are variants of the above structure.

DETAILED DESCRIPTION

In some embodiments, the systems and methods described herein provideassays (e.g., a bioluminescent assay) for the monitoring and/ormeasurement of events, conditions, responses, agent concentrations, orsignals (e.g., in real time, over an extended time course, etc.). Insome embodiments, the systems and methods described herein generate abioluminescent signal (e.g., from the interaction of a substrate andprotein sensor) in real-time response to an event (e.g., that results inconversion of a pro-substrate into a substrate).

In certain embodiments, the systems and methods provided herein comprisea protein sensor and a pro-substrate. In some embodiments, the methodsdo not require cellular engineering (e.g., alteration of the geneticmaterial of the cell or introduction of genetic material into cell). Insome embodiments, the pro-substrate is capable of traversing the cellmembrane (e.g., entering a cell, exiting a cell) by any suitablemechanism (e.g., endocytosis, diffusion, passive transport, activetransport, etc.). In some embodiments, one or more processes (e.g.,chemical, enzymatic, etc.) convert the pro-substrate (e.g.,intracellularly) into a substrate for the protein sensor. In someembodiments, the pro-substrate is converted into the substrate within acell. In some embodiments, the substrate is capable of traversing thecell membrane (e.g., entering a cell, exiting a cell) by any suitablemechanism (e.g., endocytosis, diffusion, passive transport, activetransport, etc.). In some embodiments, interaction of the substrate andprotein sensor results in a detectable signal (e.g., bioluminescence).

An aspect of many embodiments described herein comprises a system forthe continuous flow of pro-substrate into cells; its conversion to asubstrate in response to an intracellular event; its release from thecell; and signal generation by the interaction of the substrate with aprotein sensor. At any given time, the signal reflects the activity ofan intracellular event as it occurs in the cell. The system performsunder conditions where the substrate is not accumulated (e.g., to asubstantially detectable degree) extracellularly, thereby allowing forreal-time measurements, and/or continuous measurements, and/orcorrelating the detectable signal to the timing of cellular events.Following conversion of the pro-substrate into the substrate by thecellular event, response, condition or agent, the substrate is eithercompletely (e.g., substantially completely) utilized by the proteinsensor or otherwise modified or degraded so as to become unusable forlater production of a detectable signal. Because the substrate does notaccumulate, any signal detected from a protein sensor is necessarily theresult of substrate that was recently converted from the pro-substrate(e.g., within the preceding: 10 minutes, 5 minutes, 2 minutes, 1 minute,30 seconds, 10 seconds, 5 seconds, 2 seconds, 1 second, or less). Inother assays in which the substrate accumulates to a significant degree,signal may be generated from the substrate at any reasonable time overthe course of the assay, thereby precluding real-time measurementsand/or correlating the detectable signal to the timing of cellularevents.

In certain embodiments, the lag time between the event, condition,response or agent of interest and the detectable signal correlatable toits magnitude, frequency, concentration, etc. is the result of theprocesses required to bring the substrate into contact with the proteinsensor (e.g., diffusion, transport across the cell membrane, etc.). Whenthe relevant cellular and/or assay conditions are maintainedsubstantially constant, constant lag times over the course of the assayallow the timing between changes in the detectable signal or particularsignal events to be directly correlated to the timing between changes orevents in the condition, response, event or agent of interest.

Various mechanisms are employed and/or exploited to ensure thenon-accumulation of substrate in the assays described herein. Forexample, all the substrate may be rapidly consumed by the protein sensor(e.g., protein sensor may be present in excess). The substrate may beinherently unstable, unstable under cellular conditions (e.g.,susceptible to protein degradation), or unstable under the assayconditions employed. Substrate may also be produced in very smallconcentrations (e.g., in comparison to the protein sensor concentration)to ensure that all the substrate is utilized. In certain embodiments, acombination of the above strategies, as well as others, are utilized toensure that substrate does not accumulate (e.g., intracellularly,extracellularly), thereby allowing real-time monitoring with the assay.

In some embodiments, the method comprises contacting a living cell witha protein sensor comprising an Oplophorus luciferase enzyme (e.g.,NANOLUC), and a pro-substrate of the protein sensor that can enter thecell (e.g., pro-furimazine or a pro-furimazine derivative). In anexemplary embodiment, the protein sensor comprises the NANOLUC enzymeand the pro-substrate comprises a pro-furimazine substrate (e.g.,Formula I) comprising an R group, wherein R is selected from: an organicmolecule, peptide, nucleic acid, etc. The R group is removed or modifiedby an enzyme, molecule or agent that may be attributed to anintracellular agent, event, or response, thereby creating a substratefor the protein sensor. Other suitable pro-substrates for an Oplophorusluciferase enzyme (or modified version thereof) may comprise incompletefurimazine (e.g., missing a chemical group or moiety) which becomes thefurimazine substrate upon addition of the missing element by a cellularevent, agent, condition, etc. In certain embodiments, irrespective ofthe identity of the pro-substrate, once converted into the substrate(e.g., furimazine or a pro-furimazine derivative), the substrate exits(e.g., diffuses from) the cell and is utilized by the protein sensor togenerate a signal (e.g., bioluminescence). Using this method, cellularresponses are measured in real-time over an extended period of time.

In some embodiments, the method comprises contacting a living cell witha protein sensor comprising a firefly luciferase enzyme (e.g., aluciferase enzyme from Photinus pyralis or Photuris pennsylvanica), anda pro-luciferin (or pro-[luciferin derivative]) substrate. Due to therelatively high stability of D-luciferin and the slow rate ofconsumption of luciferin by the firefly luciferase, in some embodiments,accumulation of substrate is prevented by providing a limiting amount ofpro-substrate (similar limits on pro-substrate concentration may beutilized in combinations with other techniques with othersubstrate/sensor pairs) and/or by providing an excess of fireflyluciferase enzyme required for rapid substrate utilization. In otherembodiments, a more quickly utilized substrate (or sensor that utilizesthe substrate more rapidly) or less stable substrate (e.g., a luciferinderivative) is utilized. In some embodiments, an agent is added orconditions are manipulated to prevent luciferin from accumulating. Insome embodiments, the pro-luciferin substrate comprises a luciferinmoiety and a blocking moiety that prevents the protein sensor fromutilizing luciferin as a substrate. In such embodiments, removal oralteration of the blocking group results in a substrate (e.g., luciferinor modified luciferin) that is utilized by the protein sensor to producea signal. In other embodiments, the pro-luciferin substrate comprises amodified luciferin. In such embodiments, alteration of the modifiedluciferin results in a substrate (e.g., luciferin or an active modifiedluciferin) that is utilized by the protein sensor to produce a signal.In some embodiments, a pro-substrate comprises an R group (e.g., organicmoiety, peptide, nucleic acid, etc.) that prevents the pro-substratefrom being utilized by the protein sensor to produce a signal. In someembodiments, the R group comprises both blocking functionality (e.g.,preventing the pro-substrate from being utilized by the protein sensor)and functional utility (e.g., capable of being removed or altered bysome intracellular agent, event, or response). The R group is removed ormodified by an enzyme or molecule that may be attributed to an event orresponse (e.g., intracellularly, extracellularly, etc.) thereby creatinga substrate for the firefly luciferase. Some suitable pro-substrates fora firefly luciferase enzyme (or modified version thereof) may compriseincomplete luciferin (e.g., missing a chemical group or moiety) whichbecomes the luciferin substrate upon addition of the missing element bya cellular event, agent, condition, etc. In certain embodiments,irrespective of the identity of the pro-substrate, once converted intothe substrate (e.g., pro-luciferin or a pro-luciferin derivative), thesubstrate exits (e.g., diffuses from) the cell and is utilized by theprotein sensor to generate a signal (e.g., bioluminescence). Using thismethod, cellular responses are measured in real time over an extendedperiod of time.

In some embodiments, the method comprises contacting a living cell witha protein sensor comprising a coelenterazine-utilizing luciferase enzyme(e.g., Renilla luciferase or Oplophorus luciferase) and apro-coelenterazine or pro-coelenterazine derivative substrate. In someembodiments, the pro-coelenterazine or pro-coelenterazine derivativesubstrate comprises a coelenterazine moiety (or coelenterazinederivative moiety) and a blocking moiety that prevents the proteinsensor from utilizing coelenterazine as a substrate. In suchembodiments, removal or alteration of the blocking group results in asubstrate (e.g., coelenterazine or modified coelenterazine) that can beutilized by the protein sensor to produce a signal. In otherembodiments, the pro-coelenterazine or pro-coelenterazine derivativesubstrate comprises a modified coelenterazine. In such embodiments,alteration of the modified coelenterazine results in a substrate (e.g.,coelenterazine or an active modified coelenterazine) that can beutilized by the protein sensor to produce a signal. In some embodiments,a pro-substrate comprises an R group (e.g., organic moiety, peptide,nucleic acid, etc.) that prevents the pro-substrate from being utilizedby the protein sensor to produce a signal. In some embodiments, the Rgroup comprises both blocking functionality (e.g., preventing thepro-substrate from being utilized by the protein sensor) and functionalutility (e.g., capable of being removed or altered by some intracellularagent, event, or response). The R group is removed or modified by anenzyme or molecule that may be attributed to a cellular event orresponse thereby creating a substrate for the coelenterazine-utilizingluciferase enzyme. Some suitable pro-substrates for acoelenterazine-utilizing luciferase enzyme (or modified version thereof)may comprise incomplete coelenterazine (e.g., missing a chemical groupor moiety) which becomes the coelenterazine substrate upon addition ofthe missing element by a cellular event, agent, condition, etc. Incertain embodiments, irrespective of the identity of the pro-substrate,once converted into the substrate (e.g., pro-coelenterazine or apro-coelenterazine derivative), the substrate exits (e.g., diffusesfrom) the cell and is utilized by the protein sensor to generate asignal (e.g., bioluminescence). Using this method, cellular responsescan be measured in real-time and/or continuously over an extended periodof time.

In some embodiments, methods described herein provide real-time and/orcontinuous monitoring of cell viability. In some embodiments, real-timemonitoring of cell viability comprises contacting a sample comprising aliving cell with a protein sensor and a pro-substrate wherein thepro-substrate is converted inside a living cell to a substrate for theprotein sensor. In some embodiments, the reducing intracellularenvironment of a living cell converts the pro-substrate into asubstrate. The signal generated by the utilization of the substrate bythe protein sensor is proportional to the number of living cells. Insome embodiments, cell viability is measured for an extended period oftime, e.g., 1, 2, 4, 24, 48, 72 hours, or more. When using assaysdescribed herein to measure cell viability, changes in the number ofliving cells are observable in real time (e.g., with a short (e.g., <10minutes, <1 minute, <10 seconds) lag time). In some embodiments, thenon-accumulation of substrate in the assays described herein results inonly presently-produced substrate being available for the proteinsensor. When the number of living cells decrease, the amount ofsubstrate also decreases, which is detectable as a decrease in thesignal generated by the protein sensor. Due to predictable lag timesbetween pro-substrate conversion and substrate utilization, changes indetectable signal can be readily correlated to the timing of changes incell viability. Likewise, increases in detectable signal correlate, inreal-time, to increases in cell number. Other assays utilizing stablesubstrates and/or allowing for substrate accumulation cannot provide thesame real-time readout of increases and decreases in cell population.

In some embodiments, the method comprises real-time measurement ofenzyme activity inside a cell, e.g., caspase, protease, reactive oxygenspecies, cytochrome P450 (CYP450) enzyme, monoamine oxidase (MAO), etc.activity. In some embodiments, the method comprises the real-timemeasurement of caspase activation within a cell, e.g., caspase-2,caspase-3/7, caspase-6, caspase-8, or caspase-9. In some embodiments,the method comprises real-time measurement of caspase activationcomprising contacting a living cell with a protein sensor and apro-substrate for the caspase enzyme. In some embodiments, thepro-substrate comprises a pro-furimazine, e.g., Formula I, wherein R isa peptide recognition sequence for the caspase, e.g., VDVAD (SEQ ID NO:3), DEVD (SEQ ID NO: 4), VEID (SEQ ID NO: 5), LETD (SEQ ID NO: 6), orLEHD (SEQ ID NO: 7). In some embodiments, the pro-substrate comprises apro-coelenterazine or pro-luciferin. In some embodiments, the methodcomprises real-time measurement of caspase activation comprisingcontacting a living cell with a protein sensor and a pro-substrate forthe caspase enzyme.

In some embodiments, the method comprises real-time measurement ofintracellular conditions (e.g., redox state, pH, etc.). In suchembodiments, a pro-substrate is provided that is sensitive to particularconditions (e.g., oxidative stress) and is converted into a substrate inthe presence of such conditions. Conditionally-sensitive pro-substratesare converted into unstable and/or non-accumulating substrates providinga real-time readout of intracellular conditions.

In some embodiments, the methods described herein can be combined withother assay methods, e.g., cell viability, cytotoxicity, reporter geneassay. In some embodiments, the method described herein identifies themost optimal time for performing such functional assays. In someembodiments, the method described herein is combined with a fluorescentassay. In other embodiments, the method described herein is combinedwith a luminescent assay. In some embodiments, the method describedherein provides a self-contained and/or homogeneous assay. In someembodiments, methods described herein can be multiplexed to detect twoor more different events, conditions, responses, and/or agents inreal-time (e.g., using two or more different signals or locations or viaother mechanisms). Examples of such multiplexing are provided inExamples 14 and 15. The present invention is not limited by thecombinations of signals and cellular events that may be utilized inmultiplex assays.

In some embodiments, the method comprises contacting a tissue or tissuesample with a protein sensor and a pro-substrate. In some embodiments,the method comprises administering a protein sensor and a pro-substrateto an animal, e.g., mouse or rat. In some embodiments, the methodcomprises contacting a bacterial cell or bacterial cell culture with aprotein sensor and pro-substrate.

In embodiments described herein, one or more of the pro-substrate andsubstrate are capable of entering and/or exiting cells. Such entry andexit may occur by any suitable mechanism including, but not limited topassive diffusion, osmosis, active transport, endocytosis, phagocytosis,exocytosis, etc. The methods described herein are not limited by themeans of various materials passing into and out of cells, and anunderstanding of what mechanisms are used or how they function is notrequired to practice the invention.

Any of the compositions useful in carrying out embodiments of thepresent invention (e.g., protein sensor, pro-substrates, buffers, cells,controls, etc.), alone or in combination with other compositions, may beprovided in the form of a kit. For example, protein sensor andpro-substrate may be provided in a kit for detection of a cellularcondition (e.g., cell viability). Kits may further comprise appropriatecontrols (e.g., negative, positive), containers, and/or detectionreagents.

In certain embodiments, the signal is detected by any suitable device,detector, apparatus, system, etc. Depending upon the detectable signal(e.g., luminescence, colorimetric, etc.), and, for example, the assayconditions (e.g., multiplex, diagnostic conditions, etc.), suitabledetectors may include, but are not limited to: CCD cameras, videocameras, photographic film, laser scanning devices, fluorometers,photodiodes, quantum counters, epifluorescence microscopes, scanningmicroscopes, flow cytometers, fluorescence microplate readers, or afluorescence microscope. In some embodiments, detection is by real-timemonitoring, end-point reads, sequential reads (e.g., every: 1 second, 2seconds, 5 seconds, 10 seconds, 30 seconds, 1 minute, 2 minutes, etc.),or any other suitable detection/monitoring scheme.

In particular embodiments, methods are provided for multiplexingdetection of different cellular events, conditions, and/or responses.Multiplex experiments may utilize end-point detection, real-timedetection, continuous monitoring etc. In some embodiments, differentdetection methods are utilized (e.g., fluorescence, luminescence). Insome embodiments, multiplex assays measure diverse cellular responses(e.g., apoptosis, reporter gene expression, metabolite assays, etc.).Examples of such multiplexing are provided in Examples 14 and 15. Thepresent invention is not limited by the combinations of signals andcellular events that may be utilized in multiplex assays.

In some embodiments, a computer-based analysis program is used tocorrelate the raw data generated by the detection assay (e.g., thepresence, absence, or amount of signal) into the concentration of anagent of interest or the magnitude, frequency and/or concentration of anevent, condition, and or response related to that agent. The data may bereported in any suitable format (e.g., raw signal data, convertedsignal, agent concentration, event frequency, cellular conditions,etc.). The data may be stored (e.g., in a computer-readable format),displayed (e.g., on a monitor), printed, reported, etc. In someembodiments, methods provide computer-assisted analysis of the data(e.g., raw or correlated).

Various modifications of the described features and embodiments will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. Although specific embodiments have beendescribed, it should be understood that the invention as claimed shouldnot be unduly limited to such specific embodiments. For example, thepresent invention finds use with protein sensors, substrates,pro-substrates, and biological systems that are not explicitly mentionedherein. Indeed, various modifications of the described modes andembodiments are intended to be within the scope of the following claims.

EXPERIMENTAL Example 1 General Assay Set Up for Continuous, Real-TimeMonitoring of an Intracellular Event

A pro-substrate and protein sensor are supplied to live cells. Thepro-substrate enters the cells and is converted to a substrate for theprotein sensor by an intracellular event. The substrate does not produceany detectable signal until it exits the cell and is utilized by theprotein sensor. The substrate is either rapidly used by the proteinsensor to produce a “flash signal” or is degraded. The substrate doesnot accumulate outside of the cells. Therefore, the continuousproduction of signal depends on the continuous flow of the substratefrom the live cells allowing the detection of intracellular eventscontinuously in real-time as they occur in live cells (FIG. 1).

Example 2 Real-Time Monitoring of Signals Produced in Live Cells

A. A549 cells were plated into wells of a 384-well plate at 2,000cells/well. 40 ng/ml NANOLUC protein sensor and 50 μM PBI-4600pro-substrate were added to the cells either together or individually.In some cases, the protein sensor and pro-substrate were pre-mixedtogether and incubated in the presence or absence of cells Luminescencewas monitored at various time points on a Tecan Infinite 500 platereader (FIG. 2A).

B. 40 uM PBI-4600 pro-substrate and 100 ng/ml NANOLUC protein sensorwere added to cell culture media. Half of the mixture was added to wellsin a 384-well assay plate that contained media only. The other half ofthe mixture was added to wells that contained 2,000 K562 cells/well.Luminescence was monitored at multiple time points on a Tecan M1000plate reader (FIG. 2B).

The results show that upon pro-substrate conversion to a substrate forthe protein sensor by live cells, the protein sensor present outside thecells interacts with the substrate to produce a signal. The results alsoshow that both components of the method described herein (the proteinsensor and the pro-substrate), in the presence of live cells, arenecessary to generate the signal (FIG. 2).

Example 3 Signal Generated by Live Cells is Proportional to the Numberof Live Cells

Jurkat or A549 cells were plated at different densities into wells of a384-well assay plate. 50 μM PBI-4600 pro-substrate and 40 ng/ml NANOLUCprotein sensor were added to the cells. After incubation at roomtemperature for 15 (A549) or 55 minutes (Jurkat), luminescence wasmeasured on a Tecan Infinite 500 plate reader.

The results show that the signal generated using the method describedherein increased proportionally with the increase in number of livecells present in the sample.

Example 4 Signal Instability

A549 cells were plated at 5,000 cells/well into wells of a 384-wellassay plate. 50 μM PBI-4600 pro-substrate and 40 ng/ml NANOLUC proteinsensor were incubated with the cells for 2 hours, and luminescencemeasured. 1% Triton X-100 was added to the cells, and luminescencemeasured 2 minutes post-addition.

The results demonstrate that the substrate/protein sensor-generatedsignal is not stable after cell death (FIG. 4).

Example 5 Measuring Cell Death in Real-Time

A549 cells were plated into wells of a 384-well plate at 1,000cells/well in the presence of 40 μM PBI-4600 pro-substrate and 100 ng/mlNANOLUC protein sensor. The cells were incubated and the luminescencesignal was monitored at various time points on a Tecan M1000 platereader. Triton X-100 was then added to a final concentration of 1%, andcompared to control cells that received media only. Luminescence wascontinually monitored.

The results illustrate that while control cells continue to convertpro-substrate to substrate and generate light, Triton X-100 cells are nolonger able to generate substrate due to cell death and a rapid decreasein signal is observed (FIG. 5).

Example 6 Comparison of Cell Viability Reagents

A549 cells were plated into wells of a 384-well plate at 1,000cells/well. Cell viability reagents were added: 1× concentration ofAlamarBlue or 40 μM PBI-4600 pro-substrate plus 100 ng/ml NANOLUCprotein sensor, and the cells incubated for 120 minutes. Then, TritonX-100 was added to a final concentration of 1%. Luminescence andfluorescence (AlamarBlue: excitation 570 nm, emission 585 nm, bandwidths5 nm) were measured on a Tecan M1000 plate reader at various time points(FIG. 6A).

A549 cells were plated into wells of a 384-well plate at 1,000cells/well. Cell viability reagents were added: 1× concentration ofAlamarBlue or 40 μM PBI-4600 pro-substrate and 100 ng/ml NANOLUC proteinsensor. The cells were then incubated, and viability monitored atvarious time points as described above. Triton X-100 was then added to afinal concentration of 1%, and cell viability was continually monitored(FIG. 6B).

The results demonstrate that the assay described herein is reactive inreal-time to changes in live cells, a result that is not reported byconventional assays (FIG. 6).

Example 7 Reagent Effect on Cell Viability

A549 cells were plated into wells of a 384-well plate at 300 cells/well.A titration of PBI-4600 pro-substrate was added to the cells along with40 ng/ml NANOLUC protein sensor. The interference of the assaycomponents on cell viability was determined by monitoring the readout ofa cytotoxicity assay (CellTox-Green, Promega Corporation) at varioustime points. At each time point, the fluorescence readout ofcytotoxicity was measured on a Tecan M1000 plate reader (excitation 485nm, emission 520 nm, bandwidths 5 nm).

The results illustrate that the presence of the reagents comprising themethod described herein have no effect on cell viability (FIG. 7).

Example 8 Measuring Cell Proliferation

A549 cells were plated into wells of a 384-well plate at multipledensities (500, 250, 125, or 62.5 cells/well). 40 μM PBI-4600pro-substrate and 100 ng/ml NANOLUC protein sensor were added to thecells. At various time points, luminescence was measured on a Tecan M200plate reader with the gas control module (set at 37° C. and 5% CO2).Readings were taken every 30 min for 72 hours.

The results show the real-time increase in signal as cells continue togrow and proliferate (FIG. 8).

Example 9 Real-Time Measuring of Drug Induced Changes in Viability

Cardiomyocyte cells were plated and grown in media containing 40 uMPBI-4600 pro-substrate and 100 ng/ml NANOLUC protein sensor in a 37° C.,5% CO₂ incubator. At various time points, luminescence was monitored ona Tecan M1000 plate reader. After multiple days of cell growth,digitonin was added to a final concentration of 200 ng/ml. Luminescencewas read continually, starting immediately after digitonin addition.

The results show that the cells are metabolically active and continue toproduce a signal for multiple days. Addition of digitonin, a known agentto induce rapid cell death, results in rapid decrease in signalconfirming the effect of the drug on live cells (FIG. 9).

Example 10 Determining the Optimal Timing of Pharmacological ResponsesA) Response Depends on Drugs

A549 cells were plated into wells of a 384-well plate at 250 cells/wellin the presence of 40 μM PBI-4600 pro-substrate and 100 ng/ml NANOLUCprotein sensor. Various drug compounds were added to the cells, andluminescence was monitored at various time points on a Tecan M200 platereader with gas control module (settings: 37° C. and 5% CO2) using thesame set of samples (FIG. 10).

B) Response Depends on Dose

A549 cells were plated into wells of a 384-well plate at 250 cells/wellin the presence of 40 μM PBI-4600 pro-substrate and 100 ng/ml NANOLUCprotein sensor. Doxorubicin was added to the cells at variousconcentrations, and luminescence monitored at multiple time points on aTecan M200 plate reader with gas control module (settings: 37° C. and 5%CO2). (FIG. 11).

C) Response Depends on Cell Type

A549, Jurkat and PC3 cells were plated into wells of a 384-well assayplate in the presence of 40 μM PBI-4600 pro-substrate and 40 ng/mlNANOLUC protein sensor. Doxorubicin (200 nM) was added to the cells, andluminescence measured at multiple time points on a Tecan M1000 platereader. (FIG. 12)

The results in A-C show that continuous, real-time monitoring of cellviability allows examination of drug compound effect at different timepoints during treatment. The time of the response will differ dependingon the drug compounds used (FIG. 10), the drug concentration (FIG. 11)and the cell type (FIG. 12). The method described herein allowsdetection of the time of the response by monitoring repeated reads usingthe same set of samples.

Example 11 Collecting Data at Different Time Points

Jurkat cells were plated at 6,000 cells/well into wells of a 384-wellassay plate in the presence of 40 μM PBI-4600 pro-substrate and 40 ng/mlNANOLUC protein sensor. Doxorubicin was added to the cells at variousconcentrations, and luminescence measured at 24, 48, and 72 hours on aTecan M1000 plate reader. IC50 values were determined by fitting thedata to a nonlinear regression, sigmoidal dose-response (variable slope)curve using GraphPad Prism version 5.03.

The results illustrate the ability of method described herein to measurechanges in drug IC50 values during treatment using the same set ofsamples. In comparison, conventional endpoint assays require a separateset of samples must be generated for each individual time point (FIG.13).

Example 12 Comparison of Pharmacological Responses

A549 cells were plated at 3,000 cells/well into wells of a 384-wellassay plate. The cells were treated with a digitonin dose curve for 24hours. MTS and CELLTITER-GLO reagents (Promega Corporation) were addedto the cells after the 24 hour treatment according to manufacturerinstructions. For the NANOLUC/4600 test wells, 50 μM PBI-4600pro-substrate and 40 ng/ml NANOLUC protein sensor were co-incubated withdigitonin for the full 24 hour time course. Luminescence was measured ona Tecan M1000 plate reader. MTS signals were measured by analyzingabsorbance at 490 nm on the Tecan M1000 plate reader.

The results demonstrate that IC50 values determined using the methoddescribed herein are in close agreement with the values obtained by wellaccepted endpoint assays (FIG. 14).

Example 13 Using the Real-Time Cell Viability Assay to Determine OptimalTime Points for Downstream Applications

In this example, the real-time cell viability assay described herein wasused to determine the optimal time point for measuring changes inNAD/NADH levels upon treatment with the NAD biosynthesis pathwayinhibitor FK866. NAD/NADH are important cofactors that regulate cellmetabolism and effect cell growth. To better understand the role ofthose nucleotides in cell function, it is important to establish therelationship between changes in NAD/NADH levels and cell survival.

K562 cells were plated into wells of a 384-well plate at 6000 cells perwell in 25 ul media containing 40 ng/ml of protein sensor NANOLUC and 40μM of pro-substrate PBI-4600 in the presence or absence of 0.5 μM FK866compound. Luminescence was measured repeatedly using the same set ofsamples. The information was used to set up endpoint NAD/NADH detectionassays (Promega Corporation) at the time point (21 hour) before cellviability was considerably compromised and the end of drug treatment.

The results show that by using the real-time cell viability assay,optimal time points for end point assays can be conveniently determinedusing one set of samples eliminating the need for setting up multiplesamples at each time point (FIG. 15).

Example 14 Multiplexing with Luminescence Assay Methods

Current approaches for multiplexing bioluminescence assays requireeither the use of special filters or the inhibition of activity of thefirst reaction before the light generated by the second assay can bemeasured. In the method described herein, the signal depends oncontinuous substrate production by live cells and is rapidly decreasedupon cell lysis which creates conditions where the luminescence signalfrom the second reaction can be measured directly without any additionalrequirements (e.g. the use of inhibitors or special filters).

A) Multiplexing Real-Time Cell Viability Monitoring with CaspaseActivation Detection

Jurkat cells were plated at 5,000 cells/well into wells of a 384-wellassay plate. Either media (caspase activation) or 40 ng/ml NANOLUC and50 μM PBI-4600 were then added to the cells. The samples were thentreated with a TRAIL titration. Cell viability and caspase activationwere monitored at multiple time points on a Tecan M1000 plate reader byfirst measuring luminescence from the cell viability assay, addingCASPASE-GLO 3/7 Assay reagents according to the manufacturer'sinstructions (Promega Corporation; quenches the signal from theviability assay), and measuring luminescence from the CASPASE-GLO 3/7Assay.

The results illustrate that induction of apoptosis by caspase activationleads to cell death. Cell death can be monitored in real-time by themethod described herein, and the induction of apoptosis can be confirmedby multiplexing with other luminescent assays such as CASPASE-GLO 3/7(FIG. 16).

B) Multiplexing Real-Time Cell Viability Monitoring with a Reporter GeneAssay

A cell titration of HEK cells stably expressing firefly luciferase wasplated into wells of a 384-well assay plate. Media (reporter gene assay)or 40 ng/ml NANOLUC and 50 μM 4600 were then added to the cells. Cellviability was first measured as previously described, and then theONE-GLO Luciferase Assay Reagent (Promega Corporation) was addedaccording to the manufacturer's instruction Luminescence from thefirefly luciferase reporter was then measured on a Tecan M1000 platereader.

The results demonstrate that the method described herein can bemultiplexed with a reporter gene assay (FIG. 17), and the expressionlevels of the reporter can be normalized to the number of live cellspresent in the sample.

Example 15 Multiplexing with Fluorescence Assay Methods

A549 cells were plated at 300 cells/well into wells of a 384-well assayplate. 40 ng/ml NANOLUC and 50 μM PBI-4600 plus CELLTOX GreenCytotoxicity dye (Promega Corporation) were added to the cells. Thecells were then treated with a digitonin dose curve and viability(luminescence) and cytotoxicity (fluorescence; excitation 485 nm,emission 520 nm, bandwidths 5 nm) were monitored over time on a TecanM1000 plate reader.

Multiplexing fluorescence assays with end point (lytic) luminescenceassays is a commonly used approach. These results demonstrate that byusing the method described herein luminescence assays can be multiplexedwith fluorescent assays in live cells (FIG. 18).

Example 16 Monitoring Drug Effect Over Extended Period of Time

A549 cells were plated at 500 cells/well into wells of a 384-well assayplate. 50 μM PBI-4600 pro-substrate, 40 ng/ml NANOLUC protein sensor andvarious concentrations of either doxorubicin or panobinostat (forgenerating dose curves) were added to the cells. Cell viability wasdetermined at various time points by measuring the luminescent signal aspreviously described.

The results demonstrate that the method described herein can be used tomonitor changes in cell viability in real-time for an extended period oftime (FIG. 19).

Example 17 Using the Method as an Endpoint Assay

A549 cells were plated at 300 cells/well in a 384-well assay plate. Thecells were treated with a panobinostat dose curve for 72 hours. Then, 50μM PBI-4600 and 40 ng/ml NANOLUC protein sensor were added to the cellsand incubated for 1 hour. Luminescence was measured on a Tecan M1000plate reader.

The results illustrate that the method described can be used in anendpoint format (FIG. 20).

Example 18 Stability of the Pro-Furimazine Substrate, PBI-4600

PBI-4600 (50 μM) was incubated in RPMI cell culture media at roomtemperature (RT) or 37° C. for increasing time points, up to 190 hours.HPLC analysis was performed to determine the purity of PBI-4600 atvarious time points.

The results demonstrate that PBI-4600 maintained greater than 90% purityfor up to 190 hours (FIG. 22). The pro-substrate was not broken down orotherwise found to be unstable under these conditions.

Example 19 Protein Sensor Stability

A549 cells were plated at 250 cells/well into wells of a 384-well assayplate. 50 μM PBI-4600 pro-substrate and 40 ng/ml NANOLUC protein sensorwere added to the cell culture and incubated for 72 hours. Afterincubation, 40 ng/ml NANOLUC protein sensor was added to the cellculture, and the signal was monitored 1.5 hours later.

The results demonstrate that the NANOLUC protein sensor added at timezero was still sufficient to generate a strong signal 72 hours later(FIG. 23). The addition of new NANOLUC protein sensor did not furtherincrease this signal. Therefore, NANOLUC was stable in cell cultureconditions for at least 72 h.

Example 20 Pro-Substrate PBI-4601, Coelenterazine Derivative

A549 (200 cells/well) or Jurkat (6250 cells/well) cells were plated intowells of a 384-well assay plate. 50 μM PBI-4601 pro-coelenterazinesubstrate and 40 ng/ml NANOLUC protein sensor were added to the cells.Luminescence was measured on a Tecan M1000 plate reader at various timepoints.

The results show real-time monitoring of cell proliferation usingPBI-4601, a pro-coelenterazine substrate (FIG. 24).

Example 21 Real-Time Monitoring of Caspase Activation

A549 cells were plated at 1,000 cells/well into wells of a 384-wellassay plate. 50 μM PBI-3741 (a pro-furimazine caspase substrate) and 40ng/ml NANOLUC enzyme were added to the cells. To induce apoptosis,staurosporine was added to the cells at a 4 μM final concentration. Theinduction of apoptosis and activation of caspase were measured inreal-time. The increase in pro-substrate to substrate conversion uponcaspase activation was detected by the increase in luminescence duringthe time of staurosporine treatment using the method described herein.

The results demonstrate that the method described herein can be used tomonitor in real-time caspase activation (FIG. 25).

Example 22 Real-Time Monitoring of Reactive Oxygen Species (ROS)

A549 cells were plated at 1,000 cells/well into wells of a 384-wellplate and incubated overnight. Then, either 10, 25, or 50 μM of apro-furimazine hydrogen peroxide substrate, PBI-4759, and 40 ng/mlNANOLUC protein sensor were added to the cells. Menadione (200 nM), aknown ROS inducer, or a vehicle control (DMSO) was added to the cells.Luminescence was monitored over time using a Tecan M1000 plate reader.As menadione-induced hydrogen peroxide production in cells, the increasein luminescence above background was measured, and the maximum signalwas reached at 5 hours after addition of menadione.

The results demonstrate that the method described herein can be used tomonitor in real-time reactive oxygen species (FIG. 26).

Example 23 Using the Method Described Herein in Bacteria Cells

Escherichia coli (ATCC25922) were grown at 37° C. overnight. Theovernight culture was then diluted 50-fold in fresh media and incubatedfor several hours to reach log phase. Samples of the culture wereserially diluted in media into wells of a 96-well plate in the presenceof 40 ng/ml NANOLUC protein sensor and 50 μM PBI-4600 pro-substrate(dilution factors “1/32, 1/64, etc.”). The bacterial growth was measuredby monitoring increase in bioluminescence at multiple time points usingthe Tecan M1000 plate reader.

The results show that the method described herein can be used with livebacterial cells to monitor their growth in real-time (FIG. 27).

Example 24 Monitoring Cell Viability in Tissue

Tissue samples are perfused with PBS containing heparin to remove bloodand clots, and the weight determined Approximately 10 mg of tissue ishomogenized in 1-2 ml PBS containing 2 mM EDTA. The tissue extract iscentrifuged, and the supernatant collected. The tissue extract istitrated into wells of a 96-well plate along with 50 μM PBI-4600 and 40ng/ml NANOLUC enzyme. Luminescence is measured at various time pointsusing a plate reader luminometer.

1. A method of detecting an intracellular event, condition, or responsecomprising: detecting a signal generated by the interaction of asubstrate and a protein sensor; wherein the substrate is the product ofan interaction of the pro-substrate with an agent associated with theintracellular event, condition, or response; wherein the presence of thesignal indicates the occurrence of the intracellular event, condition,or response in real time; wherein the absence of an agent associatedwith the intracellular event, condition, or response results in thereal-time absence of the signal; and wherein the magnitude of the signalcorrelates with the amount of the agent.
 2. The method of claim 1,wherein the detecting comprises one or more of: real-time monitoring,continuous monitoring of said signal, and continuous monitoring ofintracellular events in live cells over extended period of time.
 3. Themethod of claim 1, wherein the pro-substrate is stable and continuouslyenters the cell.
 4. The method of claim 1, wherein the pro-substrate isconverted to a substrate by an intracellular event, condition orresponse and continuously exits the cell.
 5. The method of claim 1,wherein the substrate generates a signal by interaction with a proteinsensor placed outside the cell.
 6. The method of claim 1, wherein thesubstrate is unstable.
 7. The method of claim 1, wherein the substratedoes not accumulate extracellularly following the interaction of thepro-substrate with the agent associated with the intracellular event,condition, or response.
 8. The method of claim 1, wherein substantiallyall of the substrate generated by interaction of the pro-substrate withthe agent associated with the intracellular event, condition, orresponse is either degraded or utilized by the protein sensor.
 9. Themethod of claim 1, wherein the signal generated by the interaction ofthe substrate and the protein sensor only persists when substrate iscontinually produced by interaction of the pro-substrate with the agentassociated with the intracellular event, condition, or response.
 10. Themethod of claim 1, wherein increases and decreases in the concentrationof the agent associated with the intracellular event, condition, orresponse result in corresponding increases and decreases in the signalgenerated by the interaction of the substrate and the protein sensor.11. The method of claim 1, wherein the signal is a real-time signal. 12.The method of claim 1, wherein the signal is a non-accumulated signal.13. The method of claim 1, wherein the substrate is capable of exitingthe cell.
 14. The method of claim 1, wherein the protein sensor isprovided extracellularly.
 15. The method of claim 14, wherein theprotein sensor is incapable of entering the cell.
 16. The method ofclaim 15, wherein the interaction of the substrate and the proteinsensor occurs extracellularly.
 17. The method of claim 1, wherein thelive cells are not engineered to express the protein sensor.
 18. Themethod of claim 1, wherein the protein sensor retains activity over thetime scale of the assay.
 19. The method of claim 1, wherein the proteinsensor comprises a luciferase enzyme.
 20. The method of claim 19,wherein the luciferase enzyme comprises an Oplophorus, beetle, Renillaor Gaussia luciferase.
 21. The method of claim 20, wherein theluciferase enzyme comprises an Oplophorus luciferase with enhancedprotein stability, enhanced bioluminescence, and/or enhanced signalstability.
 22. The method of claim 19, wherein the pro-substratecomprises a pro-furimazine, pro-furimazine derivative,pro-coelenterazine, pro-coelenterazine derivative or analog,pro-luciferin, or pro-luciferin derivative.
 23. The method of claim 1,wherein the agent comprises a caspase, a protease, a reactive oxygenspecies, a cytochrome P450 (CYP450) enzyme, or monoamine oxidase (MAO),reductase, dehydrogenase, or oxidoreductase.
 24. A kit for detecting anintracellular event or response, the kit comprising: a) a proteinsensor, wherein the protein sensor generates a detectable signal uponinteraction with a substrate, and b) a pro-substrate, wherein thepro-substrate is capable of entering a cell, wherein interaction of thepro-substrate with an agent associated with the intracellular event,condition, or response converts the pro-substrate into a substrate forthe protein sensor.
 25. A method of detecting an event, condition, orresponse comprising: detecting a signal generated by the interaction ofa substrate and a protein sensor; wherein the substrate is the productof an interaction of the pro-substrate with an agent associated with theevent, condition, or response; wherein the presence of the signalindicates the occurrence of the event, condition, or response in realtime; wherein the absence of an agent associated with the event,condition, or response results in the real-time absence of the signal;and wherein the magnitude of the signal correlates with the amount ofthe agent.